First, I bootstrapped pkgsrc into the directory /usr/pkg/2006Q4.
$HOME/proj/pkgsrc/bootstrap/bootstrap --prefix=/usr/pkg/2006Q4 --unprivileged --compiler=sunpro
mkdir /usr/pkg/2006Q4/etc
mv work/mk.conf.example /usr/pkg/2006Q4/etc/mk.conf
Then I made a backup of that directory. You can never know. :)
cd /usr/pkg && gtar cfz 2006Q4.tar.gz 2006Q4
Since in a previous try, the pre-build program had removed the pkgsrc directory, I just commented it out by inserting an exit 0 in line 2. Since now there is no danger that anything in that directory is deleted, I could edit the mk.conf file and adjust some local settings.
#/usr/pkg/2006Q4/mk.conf

I began to setup the bulk build environment in a screen.
$ screen -S
TOOLS_PLATFORM.awk= /usr/pkg/current/bin/nawk
TOOLS_PLATFORM.sed?= /usr/pkg/current/bin/nbsed
FETCH_CMD= /usr/pkg/current/bin/ftp
TOOLS_PLATFORM.pax?= /usr/pkg/current/bin/pax
TOOLS_PLATFORM.tar?= /usr/pkg/current/bin/tar
TOOLS_PLATFORM.mtree?= /usr/pkg/current/sbin/mtree
Luckily, bin/bmake is not registered as belonging to any package, so it hasn't been removed yet.
The bulk build also tried to deinstall the infrastructure packages, so I had to protect them against that: